SSLv3 uses an early form of HMAC for message authentication functions (we will denote this MAC as SSL3-MAC for brevity). A critical point of the security of HMAC (and SSL3-MAC) is that the each of the transformed keys (termed ikey and okey) is exactly B bytes long, where B is the input size of the hash function (for the MD5 and SHA-1 hash functions, B = 64).
SSL3-MAC is different from standard HMAC as follows: instead of XOR'ing two fixed B-byte long strings against the input key to form ikey and okey, it appends two different byte strings, whose length depends on the block size of the hash function, and the size of the key. In SSLv3, the size of the key is fixed to be the same as the output size of the hash function.
For MD5, the key is 16 bytes long and the padding strings are 48 bytes long, leading to a total size of ikey and okey of 64 bytes (which is equal to B). However, for SHA-1, the padding strings are specified to be only 40 bytes long, meaning the length of ikey and okey are both just 60 bytes (with the 20 byte key), 4 bytes short of B. This is not merely an error in the specification, since at least one well known and very widely used implementation of SSLv3 uses exactly this definition for the authentication code.
It would seem this would, at least potentially, mean that SSL3-MAC with SHA-1 can be attacked faster than would be expected; in particular, it may be faster to attack than HMAC with SHA-1, and could possibly be faster to attack than SSL3-MAC with MD5 as well.
However, there are some factors which mean that an attack is, if not impossible, at least hard to find. In particular, SHA-1 is a quite good hash function, and its method of expanding its input words would make an attack of this sort, based on only the last 4 bytes of the input block, quite hard to exploit. Also, the HMAC construction, which SSL3-MAC shares, seems to be able to tolerate this error to at least some degree. The original HMAC papers claimed that there were significant security reductions if the padding did not pad to a full blocksize, but there was relatively little explanation of why.
Lastly, the ascendance of TLS (which uses HMAC, and thus is not affected by the problem) over SSLv3 means that the issue is rapidly disappearing in any case.